Planetary Atmospheres (Extraterrestrial)

The gaseous envelopes of planets and moons beyond Earth, from the crushing carbon-dioxide blanket of Venus to the hazy nitrogen-methane air of Titan.

Definition

Extraterrestrial planetary atmospheres are the gaseous envelopes surrounding planets and moons other than Earth, studied through their composition, structure, dynamics, and climate.

Scope

This topic covers the atmospheres of bodies other than Earth: their composition and origin, vertical thermal structure, clouds and hazes, and large-scale circulation and weather. It spans the thick atmospheres of Venus, Mars, the giant planets, and Titan, as well as thin and exotic atmospheres elsewhere, and it treats the radiative, dynamical, and chemical processes that govern them. The comparative approach uses this diversity to test general theories of atmospheric structure and climate.

Core questions

• What sets the composition and vertical structure of an atmosphere?
• How do rotation, heating, and condensation organize atmospheric circulation and weather?
• How do greenhouse effects and clouds control surface and atmospheric temperatures?
• What chemical processes form the hazes and clouds seen on other worlds?

Key theories

Radiative-convective equilibrium

The balance between radiative heating and cooling and convective overturning sets an atmosphere's temperature profile and the strength of its greenhouse warming.

Atmospheric general circulation

Differential heating and planetary rotation drive circulation patterns, from Hadley-like cells and superrotation on slow rotators to banded zonal jets on the rapidly rotating giants.

Mechanisms

Sunlight absorbed at depth and infrared radiation emitted from above set an atmosphere's temperature structure, with greenhouse gases trapping heat near the surface. Uneven heating and rotation drive winds and circulation cells, while photochemistry and condensation produce clouds and hazes that feed back on the radiation balance.

Clinical relevance

Comparing atmospheres across the Solar System reveals the general principles of climate and circulation, providing essential context for understanding Earth's atmosphere and for interpreting exoplanet atmospheres.

History

Spacecraft transformed the study of other atmospheres: Venera and Pioneer Venus probed Venus's dense atmosphere, Viking and later landers and orbiters characterized the thin Martian air, the Galileo probe sampled Jupiter's atmosphere, and Cassini-Huygens revealed Titan's thick nitrogen-methane atmosphere and surface, establishing comparative planetary climatology.

Debates

Origin of Venus's atmospheric superrotation

Why Venus's atmosphere rotates far faster than the slowly spinning planet beneath it remains incompletely explained despite extensive observation and modeling.

Key figures

• Fredric Taylor
• Andrew Ingersoll
• James Pollack
• Carl Sagan

Seminal works

• taylor2010
• ingersoll2013

Frequently asked questions

Which moon has a thick atmosphere?

Saturn's moon Titan has a dense nitrogen atmosphere thicker than Earth's, complete with methane clouds and rain that carve rivers and fill lakes on its surface.

Why is studying other atmospheres useful for Earth?

Other planets act as natural experiments with different temperatures, compositions, and rotation rates, helping test the general physics behind Earth's climate and weather.